1. Field of the Invention
The present invention relates to semiconductor devices.
2. Description of the Prior Art
In recent years, with advances in information communication equipment, semiconductor devices such as large-scale integrated (LSI) circuit chips are becoming more strict year by year in processing performance required therefore. Under the circumstances, attempts are made to speed up the operating rates of transistors. Until today, such speed-up of transistor operation has been advanced mainly based on miniaturization or shrinkage of the fabrication sizes of gate electrodes. Recently, the gate length is shrunk to 0.1 micrometer (μm) or less. Another requirement is that the film thickness of a gate electrode is much reduced to about 20 nanometers (nm) in cases where silicon is used as the main constituent material thereof. The influence given to signal transmission delay by an increase in resistance resulted from a decrease in line width of gate electrode—this has not specifically been considered as a serious problem in past days—is becoming more significant to an extent that such is no longer negligible for achievement of ultrahigh-speed transistor operations. In order to resolve this problem, it is inevitable to employ a certain material which promises high-speed performance and low power consumption capabilities while at the same time being large in familiarity with currently established silicon process architectures. One of recently challenged approaches to discovery of such material satisfying the above-noted requirements is to make use of silicon germanium for gate electrodes. This material is germanium-added silicon, wherein the germanium is such that its electron mobility is as high as approximately 2.6 times greater than that of silicon, while the hole mobility is about 4.2 times greater than that of silicon. Another approach presently considered successful is to use a method for increasing the magnitude of a drain current by using the silicon germanium—this is greater in lattice constant than silicon—for a foundation film underlying a channel region (i.e. the so-called “channel underlayer” film) to thereby give a strain to the silicon. This is relied upon the fact that giving such strain to the silicon underlying a gate dielectric film results in an increase in electron mobility.
Unfortunately, since metal oxide semiconductor (MOS) transistor microfabrication processes typically require addition of thermal processing steps at high temperatures after having fabricated gate electrodes and/or channel underlayer films, the gate electrodes and channel underlayer films made of the silicon germanium can often experience unwanted diffusion and aggregation or cohesion of germanium occurring due to the thermal processing. This would result in a change in composition of the silicon germanium, which leads to a decrease in mobility and a change in work function of silicon germanium. This in turn pauses a problem such as a variation or fluctuation in threshold voltage values of MOS transistors. Another penalty is that any sufficient strain is no longer given to channels, resulting in occurrence of a problem as to a decrease in drain current.
A remedy for this problem is found, for example, in JP-A-2000-269501, which discloses therein one form that fabricates a carbon-contained silicon film at the upper surface of a channel underlayer film made of silicon germanium chosen as its main constituent material. Several forms which dope or implant carbon atoms into silicon-germanium gate electrodes are disclosed in Japanese patent disclosure documents, such as JP-A-2000-77425, JP-A-2002-134741, JP-A-2002-184993 and others.
However, the inventors of the invention as disclosed and claimed herein have found out that the use of the techniques for merely doping carbon in the way taught by the forms disclosed in the above-identified prior art documents suffers from a risk: an increase in leakage current in accordance with the doping amount thereof.
It is therefore an object of the present invention to provide a semiconductor device which is high in performance and low in resistance and which has an ability to suppress leakage currents.